1. Field of the Invention
The present invention relates to an optical cable suitable for a long distance communication and a manufacturing method of such an optical cable.
2. Description of the Related Art
In an optical communication system using an optical cable, an optical signal generated from an optical source is transmitted to a distant place through optical fibers within the optical cable and is received by an optical receiver. To transmit the optical signal for a long distance, the optical signal transmitted through the optical fiber must be large in power, the optical fiber, which is a transmitting medium, must be low in optical loss, and the optical receiver must be high in sensitivity. However, in the conventional optical cable, the optical power transmitted from the optical source through the optical fiber is reflected within the fiber due to stimulated Brillouin scattering (SBS), so that, even if the optical power is ever being increased, the optical power which can be transmitted through the optical fiber (hereinafter referred to as transmitted power) will soon reach at its upper limit. FIG. 1 shows an example of the experimental results of the transmitted power within the fiber, in which a laser beam having a spectral width (FWHM) of 73 kHz is introduced into a single mode optical fiber of a quartz group with a 125 micron diameter. Even if the optical power of the optical source is ever being increased, the transmitted power will be saturated at about 1 mW and will not be increased any more. FIG. 1 also shows that the curve of a back-scattering power will sharply increase due to the transmitted power being saturated. The limitation imposed upon the transmitted power due to the stimulated Brillouin scattering will be a great hindrance in increasing a transmission distance.
In the conventional optical cables for a long distance communication, the optical fibers are generally designed such that they will surely be prevented from being stretched or strained, because they are mechanically fragile. What follows is a literature which is written from a viewpoint of reducing strain: S. Hatano etc., "Multi-hundred-fiber cable composed of optical fiber ribbons inserted tightly into slots," Proceeding of IWCS, 1986. This literature happens to show an optical cable with a double helix structure. The detail of the double helix structure is stated in the literature, W. Katsurashima "Characteristics of 1000-Fiber Optical Cable Composed of Tape-Slot Type Optical Fiber Units," Technical digest of IOOC'89, Paper No. 1983-10. According to this literature, the double helix structure in the conventional optical cable is designed such that the fibers move freely within the cable so as to relieve a bending strain which may occur within each of the fibers when laying the cable.
In short, the prevention of strain is a generally accepted idea in the conventional optical cables, but it is not ever proposed to positively provide an optical cable with strain, which is uneven in the longitudinal direction of the optical cable, for increasing the Brillouin gain bandwidth, a critical input power and the transmission distance.
In addition, it is also generally known to bind coated fibers together with adhesive resin so as to form an optical fiber unit. Such a technique is described, for instance, in the following literature: N. Yoshizawa et al., "Design and Characteristics of Optical Fiber Unit for Submarine Cable," IEEE, JLT, Vol. LT-3, No. 1, 1985. In this literature, the optical fibers and the cable core are closely adhered to one another with adhesive resin, so that the fibers and the core elongate or contract as an integral optical unit and water is prevented from entering into the optical fiber unit. The water propagation blocking property of this unit is described in the following literature: N. Yoshizawa et al., "Water Propagation Blocking Properties of Submarine Optical Fiber Cables," Electronics and Communications in Japan, Part 2, Vol. 70, No. 7, 1987.
In contrast, in the present invention, adhesive resin is used to bind the coated optical fibers, each positively given strain, to a center member to prevent the strain having been given to each fiber from being averaged in the longitudinal direction of each fiber, which technique is not disclosed in any of the above references.